shared/course_components_Section_learner_problem_view.rst has the following note:
Problems can be *open* or *closed*. Closed problems, such as problems whose due dates are in the past, do not accept further responses and cannot be reset. Learners can still see questions, solutions, and revealed explanations, but they cannot submit responses or reset problems.
This text appears in note form, far down in the topic, kind of out of the blue. The idea of open and closed problems should occur earlier and be more obviously connected to the other concepts in the topic.